After entering password in graphical login screen, just get the login screen displayed again.
No .xsession-errors file.
Same result with "GNOME" and "GNOME on Xorg" sessions, but "Gnome (Wayland)" session works.
Also running startx from virtual terminal works (shows my Plasma session.)

Found some AVC logs relating to sddm-helper and .Xauthority and .xsession-errors.
If I "sudo setenforce 0" then I am able to login to the Plasma session.

Found that ~/.Xauthority had incorrect security context.
Running restorecon had no effect. Used chcon to change the security context to xauth_home_t
and the graphical login into Plasma session works now.